Subject: RE: [IBIS] Operating conditions and sweep ranges
From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Fri Mar 07 2003 - 09:09:06 PST
Gaspard,
The problem is that the IBIS specification was not written well enough
to define and get this detail clearly across. There is also another
rule that says that none of the tables can have NA as their first or
last data point. So if you sweep strictly according to the -Vcc to
2Vcc rule, you would end up with NAs in the typical and minimum curves.
What this means is that you have to sweep each case across the same range.
You have two choices: You can sweep all cases with the maximum supply
values that will extend the range for the typ and min slightly, or just
go with the typ supply value which cuts the max a little short, and adds
to the min case a bit. I wouldn't recommend to use the min supply value,
because that is cutting both typ and max curves short.
Another area that the spec overlooked is that there are cases when the
signal swing is much smaller than the rail voltages, as in GTL, DDR, LVDS,
etc. For example, we can have a GTL signal that swings from about 0.4
to 1.5 volts, while its supply is 3.3 volts.
The reason the -Vcc to 2Vcc rule was established is based on the reflection
phenomena of T-lines. If you have a full reflection, the signal doubles
at the unterminated end. We wanted to make sure that the IV curves cover
the entire signaling range. So if you double the signal of a CMOS buffer
that switches rail to rail, then provided that your buffer is strong enough
to launch a rail to rail incident wave (i.e. it is a superconductor) than
doubling that wave you could have a signal at the end of the (lossless)
T-line that could reach -Vcc or 2Vcc. These are the extreme possible
values which will actually never happen for obvious reasons.
Now, going back to a low voltage swing buffer, like the GTL example above,
you really shouldn't have to cover -Vcc to 2Vcc (-3.3 to 6.6 V), because
based on the signaling levels you would see at most -1.5 to 3.0 V if it
was an unterminated system, but GTL is actually parallel terminated, so
you would probably never even see that much.
Even though the spec says -Vcc to 2Vcc, the IBIS checker program doesn't
reject models (as far as I know) if you don't cover that range exactly.
I usually recommend that the range should make sense based on the argument
I described here that considers the signal swing as the decision factor.
I hope this helps,
Arpad Muranyi
Intel Corporation
============================================================================
-----Original Message-----
From: Gaspard Thaller [mailto:gthaller@psi-e.com]
Sent: Friday, March 07, 2003 1:39 AM
To: ibis@eda.org; ibis-users@eda.org
Subject: [IBIS] Operating conditions and sweep ranges
Hello all,
All I/V curves of an IBIS model are generated for a given voltage swing
range, depending on the rail power supply value.
That is [-Vcc, 2Vcc] for [Pulldown] and [Pullup] tables, [Vcc, 2Vcc] for
[Power Clamp], etc...
The sweep range is defined as a single variable in s2ibis2 C source
code, and the tables are all given for the same voltage swing (mainly
based on typical voltage values), without regard to the voltage
variations due to operating conditions (min, max).
I mean you got Vcc(typ), Vcc(min), Vcc(max), it should define a
different voltage swing value for each operating condition.
What does the IBIS spec says about that?
Are all s2ibis2 simulations done with the same voltage swing values?
How are the current data gathered and grouped under the same sweep
range, when the sweep ranges are supposed to be different?
I hope I made myself clear.
Gaspard
-----------------------------------------------------------------
|For help or to subscribe/unsubscribe, email majordomo@eda.org
|with the appropriate command message(s) in the body:
|
| help
| subscribe ibis <optional e-mail address, if different>
| subscribe ibis-users <optional e-mail address, if different>
| unsubscribe ibis <optional e-mail address, if different>
| unsubscribe ibis-users <optional e-mail address, if different>
|
|or email a request to ibis-request@eda.org.
|
|IBIS reflector archives exist under:
|
| http://www.eda.org/pub/ibis/email_archive/ Recent
| http://www.eda.org/pub/ibis/users_archive/ Recent
| http://www.eda.org/pub/ibis/email/ E-mail since 1993
-----------------------------------------------------------------
|For help or to subscribe/unsubscribe, email majordomo@eda.org
|with the appropriate command message(s) in the body:
|
| help
| subscribe ibis <optional e-mail address, if different>
| subscribe ibis-users <optional e-mail address, if different>
| unsubscribe ibis <optional e-mail address, if different>
| unsubscribe ibis-users <optional e-mail address, if different>
|
|or email a request to ibis-request@eda.org.
|
|IBIS reflector archives exist under:
|
| http://www.eda.org/pub/ibis/email_archive/ Recent
| http://www.eda.org/pub/ibis/users_archive/ Recent
| http://www.eda.org/pub/ibis/email/ E-mail since 1993
This archive was generated by hypermail 2b28 : Fri Mar 07 2003 - 09:17:21 PST